1. Field of the Invention
The present invention relates to a method and apparatus for determining the location of a mobile radio transmitter, especially a mobile radio transmitter positioned in the service area of a cellular communications telephone system.
2. Description of Related Art
Many attempts have been made over time to meet a recognized need for quickly and accurately determining the position of a mobile radio transmitter. A radio transmitter may typically be attached to a vehicle which would enable the vehicle to be localized for purposes of protecting endangered cargo or persons, controlling the deployment of delivery trucks in an urban area or any other of a number of applications.
Several localization systems are presently commerically available. Some of the systems use navigational instruments such as ring laser devices. Others use magnetic field sensors that are sensitive to the earth's magnetic field. Yet another type uses radio beacons such as the LORAN-C system. While the systems perform satisfactorily, they are not suited for consumer use due to their inherent complexity and cost as well as the need for frequent reinitialization or calibration.
Another system for determining geographic position involves a small radio receiver to receive signals from the global positioning (GPS) system. The GPS system offers the potential to produce a three-dimensional position anywhere on the surface of the earth to within a few feet. Radio receivers for the GPS system have become relatively inexpensive. The GPS system, however, has yet to be deployed because no launch vehicle exists that can both reliably and economically launch commercial communication satellites on schedule. Further, present satellite communication systems provide coverage only for large geographic areas. Even if the GPS system does become operational, no satellite system will exist that can record the position of a large number of terrestrial vehicles in a small geographic area such as a city. While proposals have been made to orbit communications satellites that can service small geographic areas, no satellite system presently scheduled for launch in at least the next decade would permit reusing radio frequency channels. Thus, any satellite-based vehicle location system would be inefficient in utilizing limited radio frequency space.
Several attempts have been made over time to use terrestrial based radio direction finding and positioning systems. One type of radio positioning system measures the time required for a radio signal to travel between a mobile transmitter station and fixed antenna locations. Time difference measurements are obtained by comparing the wide band signal wave forms transmitted from the mobile station with some form of pulsed amplitude modulation or specific coding modulation so that the timing resolution and related position resolution is proportional to the inverse of the signal bandwidth. Yet another radio positioning system uses time difference measurements obtained by comparing narrow band signal wave forms to obtain a difference in the phase of the received signal. The radio frequency signals have a wavelength comparable to the separation of the antenna sites and the ability to resolve the location of the mobile transmitter station is proportional to the wavelength of the signal. Either radio positioning system, however, requires synchronizing separated antenna sites. The synchronization requirement may be overcome by adding a special, known wave form to the radio signal. The waveforms received at each receiver, however, must be compared to determine the position of the mobile transmitter station. The common waveforms must be impressed on the signal by special equipment. Further, the waveforms must not be distorted by any intervening interference.
A simpler method of determining the location of a mobile radio transmitter station involves measuring the angle of arrival of a radio signal at a number of fixed locations and then determining the area in which all the direction angles cross. One way to determine the direction angle is to electronically compare the difference in phase of the radio signal that is received by different antenna elements at a receiver site. Positioning the two antenna elements approximately 1/2 wavelength apart produces a narrow band intersignal phase difference which is proportional to the sine of the angle at which the signal is received.
Since most vehicles operate in an urban setting, it is highly desirable that any vehicle location system operate in an urban environment. A moving vehicle in an urban environment, however, seldom has a direct line-of-sight path to a receiver station. Rather, the propagation path contains many obstacles in the form of buildings and other structures, hill, and other vehicles which may be either landborne or airborne. The absence of a unique propagation path between the vehicle and the receiver station causes the instantaneous signal strength of any radio signal emitted from the vehicle to be highly variable at the receiver station. Indeed, it is known that the main propagation features of a radio signal in an urban environment are produced by multipath interference and shadowing of the direct line of sight path by intervening features of the terrain. Multipath interference typically corresponds to so called Rayleigh signal fades. The signal fades occur because of plane wave interference and are separated by a ground distance of approximately one half wavelength apart. Multipath interference produces irregularly varying patterns of constructive and destructive interference as the mobile radio transmitter station moves through the service area which causes the radio frequency signal to fluctuate in amplitude, travel time and propagation direction. The exact position of any maxima or minima also depends on the wavelength of the radio frequency signal. Data which is transmitted during a deep fade typically is lost. Thus, multipath interference produces a complicated pattern of signal distortion which is an inherent characteristic of RF transmissions in an urban environment.
It is known that shadowing and multipath interference does not distort the frequency or the phase of a radio frequency signal as much as its amplitude and travel time. Thus, the relative insensitivity of frequency modulation to multipath interference has encouraged its use in cellular telephone systems which operate in, for example, the 800-900 MHz frequency bands. Shadowing and multipath interference nevertheless cause the apparent position of the mobile radio transmitter to randomly change with time so as to severly limit the accuracy and, hence, the applicability of the above mentioned phase difference technique of radio position locating to solving the problem of localizing a large number of vehicles in an urban environment.